EUV Radiation Generating Device Including a Beam Influencing Optical Unit

ABSTRACT

An extreme ultraviolet radiation generating device includes a source operable to generate a first and second entrance beam, and a beam unit operable to modify at least one of a direction and a beam divergence of the first and second entrance beam, in which the beam unit includes: a beam splitter to receive the first and second entrance beam, the beam splitter being configured to reflect the first entrance beam as a first exit beam and to transmit the second entrance beam; and a mirror in the beam path of the transmitted, second entrance beam to reflect the second entrance beam to form a second exit beam that is transmitted by the beam splitter and that is at least partially superposed on the first exit beam, in which the beam unit is configured to modify an angle and/or beam divergence between the first and second exit beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority under 35U.S.C. §120 to PCT Application No. PCT/EP2014/063153 filed on Jun. 23,2014, which claimed priority to German Application No. DE 10 2013 212685.9, filed on Jun. 28, 2013. The contents of both of these priorityapplications are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to an extreme ultraviolet (EUV) radiationgenerating device having a beam influencing optical unit.

BACKGROUND

Optical units for influencing two or more beams, such as two or morelaser beams, are used in various technological areas such as, forexample EUV radiation generating devices.

SUMMARY

In general, according to one or more aspects, the subject matter of thepresent disclosure can be embodied in an EUV radiation generating devicehaving a beam influencing optical unit with which it is possible toefficiently influence beams, in particular laser beams, with respect totheir beam direction and/or their beam divergence. The EUV radiationgenerating unit can include at least one radiation source for generatinga first and second entrance beam that is at least partially superposedon the first entrance beam and that can extend in particularsubstantially collinearly with the first entrance beam. The beaminfluencing optical unit can modify the first entrance beam and thesecond entrance beam with respect to beam direction and/or beamdivergence.

Implementations of the EUV radiation generating device can include oneor more of the following features. For example, in some implementations,the EUV radiation generating device includes: a beam splitter that isconfigured to reflect the first entrance beam as a first exit beam andto transmit the second entrance beam; and a mirror element that isarranged in the beam path of the transmitted, second entrance beam andthat reflects the second entrance beam back to the beam splitter so asto form a second exit beam that is transmitted by the beam splitter andat least partially superposed on the first exit beam. The beaminfluencing optical unit can be configured to modify an angle betweenthe first and the second exit beams and/or to modify a beam divergenceof the first and/or of the second exit beam.

The EUV radiation generating device is operable to modify the beamdirection and/or the beam divergence of two partially superposed, inparticular (approximately) collinear beams that enter the EUV radiationgenerating device using two optical elements, such as the beam splitterand the mirror element. A partial superposition of beams within thecontext of this disclosure is understood to mean that their beam pathspartially intersect or that the beams partially overlap. The first andsecond entrance beams can be beams that are generated by one or moreradiation sources. The first and second entrance beams are not requiredto pass through the beam influencing optical unit at the same time;instead, it is possible for the first and second entrance beams to passthrough the beam influencing optical unit with a time offset, such as inthe case of pulsed beams. The entrance beams can be two radiationcomponents of the same beam, of which one component is reflected by thebeam splitter and the other component is transmitted by the beamsplitter. The first and second entrance beams can be mutually collinear.Irrespective of whether one radiation source or a plurality of radiationsources generate the first and second entrance beams, the beam splittersplits the two entrance beams, based on at least one optical property inwhich the two entrance beams differ, into a first radiation componentthat is reflected as the first exit beam and a second radiationcomponent that is transmitted to the mirror element.

To modify the relative beam directions of the first and second exitbeams, the beam splitter and/or the mirror element of the beaminfluencing optical unit can be arranged in different relative positionsand/or relative alignments. This allows not only the alignment of thetwo exit beams at a (typically variable) angle with respect to oneanother, but also a parallel or collinear progression of the first andsecond exit beams with respect to one another. The first and secondentrance beams can be collimated when they strike the beam splitter,although this is not required. To modify the beam divergence of thefirst and/or of the second exit beam, e.g., to generate a divergent orfocused exit beam from a collimated entrance beam, the beam splitterand/or the mirror element can be configured to be deformable.

In the context of the present disclosure, modifying a beam direction isunderstood to include changing a quantity associated with the beam(e.g., the alignment of a first exit beam relative to another exit beamor a beam divergence of an exit beam) using the beam shaping opticalunit. The change can be obtained using actuators, but it is alsopossible that the beam shaping optical unit permits a manual or, ifappropriate, a predetermined change of the quantity (relative alignmentor divergence) of the beam.

The mirror element can, in some implementations, reflect the second beamcomponent completely or nearly completely to the beam splitter, e.g.,the mirror element can have a highly reflective configuration. However,the mirror element can also be configured to be partially reflective orpartially transmissive. In some implementations, the radiation componenttransmitted by the mirror element can be used, for example, for a beammeasurement. The second exit beam is generally at least partiallysuperposed on the first exit beam, e.g., the beam path or the beam pathsof the exit beams at least partially overlap, such that the dimensionsof the optical surfaces of optical elements that are arranged in thedownstream beam path of the two exit beams can be kept as small aspossible. To achieve this, the beam splitter and the mirror element aretypically arranged with a small distance with respect one another, inwhich the small distance is generally less than twice the beam diameterof an entrance beam or is less than the sum of the beam diameters of theentrance beams.

In some implementations, the beam splitter and/or the mirror element aretiltable so that the angle between the first and second exit beams canbe modified. The beam splitter and/or the mirror element can be tiltableusing actuators. Since the beam splitter and/or the mirror element aretiltable, there exists, in some implementations, particularly effectiveconfigurations for modifying the beam directions of the two exit beams.It is possible, for example, to arrange the beam splitter in a spatiallyfixed manner in the beam influencing optical unit and for the mirrorelement to be tiltable relative to the beam splitter. Alternatively oradditionally, it is possible for the beam splitter to be arranged to betiltable in the beam influencing optical unit. Here, both elements (beamsplitter and mirror element) can be mounted within the beam influencingoptical unit (for example, at a main structure of the beam influencingoptical unit) such that they are tiltable about one or more tilt axes,in which the tilt axes can be arranged, for example, such that each axispasses either through a corresponding element (for example, through thecenter of the corresponding element) or is offset with respect to thecorresponding element (for example, outside the corresponding element).

Actuators can be used to tilt the elements. In some implementations, theactuators can be controlled precisely and quickly such as, for example,servomotors or piezomotors having relatively short displacement paths.Alternatively or additionally, it is possible to manually tilt at leastone of the two elements to modify the beam directions of the firstand/or second exit beams relative to one another. In someimplementations, the element can be tilted and fixed in a respectiveangular position.

In some implementations, the mirror element is configured such that itis deformable in order to modify the beam divergence of the second exitbeam. For example, the mirror can be deformed using an actuator. Thedeformation of the mirror element allows, in some implementations, thebeam divergence of the second exit beam to be modified without changingthe beam divergence of the first exit beam. During the deformation ofthe mirror element, the optically active surface of the mirror element(e.g., the surface form of the mirror surface) is changed such that thebeam divergence of the second exit beam varies. If the mirror surface ofthe mirror element curves, for example, convexly to the outside, thedivergence of the reflected beam increases. If, on the other hand, thecurvature is reduced, in particular in a concave manner, the divergenceof the beam can decrease as well.

The deformable mirror element can be configured as a mirror that has amembrane carrier and a mirror plate membrane attached thereto. Pressure,which can be supplied, for example, from a fluid, can be applied to themirror plate membrane on its interior membrane rear side. Under theinfluence of the pressure, the mirror plate membrane curves to a greateror lesser extent or changes its geometric shape. Alternatively, it isalso possible for an actuator, which is arranged on the rear side of themirror element, to be provided to deform the mirror element, such as todeform the mirror surface. For example, the actuator can include anelectromechanical servomotor or a piezoelectric element or multiplepiezo elements. Actuators that can be controlled precisely and quicklycan be used to permit quick and precise adjustment of different mirrorsurface geometries. Such actuators can engage centrically on the rearside of the mirror surface.

In some implementations, the beam splitter is configured to beplate-shaped or plate-type, e.g., the beam splitter has a first opticalsurface on the entrance side and, opposite to the entrance side (facingthe mirror element), a second optical surface. The two optical surfaceson the opposite sides of the beam splitter do not necessarily have to beparallel to one another. For example, they can also be arranged at anon-parallel angle (for example, an acute angle), such that theplate-type beam splitter has a wedge shape. The optical surfaces can beconfigured to be planar or have a curvature. The use of a plate-shapedbeam splitter not only permits a compact construction of the beaminfluencing optical unit, but also an at least partial superposition ofthe exit beams, without the use of additional optical elements.

In some implementations, the beam splitter is configured as awavelength-selective element, e.g., the two entrance beams are eitherreflected or transmitted based on their wavelengths. For example,radiation having a wavelength of less than a specific wavelength limitcan be reflected by the beam splitter directly as a first exit beam,whereas radiation having a wavelength that is greater than thewavelength limit can be transmitted by the beam splitter and, afterreflection at the mirror element, be transmitted in turn by the beamsplitter so as to form the second exit beam. The beam splitter can havea wavelength-selective coating on the optical surface on the entranceside for wavelength selection.

In some implementations, the beam splitter is configured as an etalon.In this case, radiation transmitted by the beam splitter is restrictedto a defined wavelength or a defined (e.g., narrow) wavelength range.The first entrance beam is reflected at the etalon as a first exit beam.The effect of an etalon is based on the basic principle of theFabry-Perot interferometer, in which typically two mutually parallelplanar mirror surfaces arranged with a small distance (for example, onthe order of magnitude of micrometers) are used to form a cavity. Suchan etalon is restricted to transmitting radiation at a wavelength or anarrow wavelength range that meets the resonance condition. A substratehaving, on a first and a second side surface, a (e.g., partially)reflective coating, such as a thin plane-parallel plate, can be used asan etalon. At one side surface, the coating can be, for example, in theform of a stack of alternating high-refractive and low-refractivequarter-wave layers. The coating can be mounted on a substrate that ismade non-reflective on the rear side and having a thickness thatcorresponds to, for example, a multiple of a half-wave layer. A coatingapplied on the other side surface can have another stack of alternatinghigh-refractive and low-refractive quarter-wave layers. The terms“half-wave” or “quarter-wave” layers here relate to the opticalthickness of the layers, which corresponds to half or a quarter of thecentral (resonant) wavelength of the etalon. It is also possible to useother types of etalons such as, for example, etalons referred to asair-spaced etalons, in which two thin flats are kept at a specifieddistance with respect to one another by spacers. It is furthermorepossible to configure the distance of the flats with respect to oneanother such that the distance is adjustable for setting the resonancecondition (e.g., to set the wavelength or the wavelength range that istransmitted by the etalon). The first entrance beam is reflected by theetalon, and the second entrance beam is transmitted and reflected at themirror element.

In some implementations, the beam splitter is configured as a polarizingbeam splitter. The beam splitter can split the incoming radiation (thefirst and second entrance beams) based on their polarization properties.As an example, radiation having a first polarization direction can bereflected by the beam splitter directly as the first exit beam, whereasradiation having a second polarization direction that is typicallyperpendicular to the first polarization direction is transmitted by thebeam splitter. The entrance beam having the first polarization directionand the entrance beam having the second polarization direction can betwo perpendicularly polarized components of the same beam that entersthe beam influencing optical unit. The beam can be generated by oneradiation source and can be polarized, for example, elliptically,circularly or linearly. In the case of a linearly polarized beam, thepolarization direction typically may not extend in the plane ofincidence or perpendicular to the plane of incidence since otherwise theincoming radiation is either only reflected or only transmitted.

In some implementations, an optical surface on the beam entrance side ofthe beam splitter and/or a side of the beam splitter facing the mirrorelement have a curvature for producing a predetermined divergence of thefirst and second exit beams. Thus, in some implementations, it isadvantageously possible to change the first exit beam in terms of itsdivergence. Due to the optical surface curvature of the entrance sideand/or of the side facing the mirror element (e.g., a convex curvature),the beam divergence of the first exit beam can be influenced such thatit takes a specific (e.g., increased) value. It is also possible toconfigure the curvature of the optical surfaces (the entrance sideand/or the side facing the mirror element) such that they are adjustable(e.g., deformable), as described herein, with respect to the mirrorelement. In such implementations, the beam divergences of both the firstand the second exit beam can be varied independently of the mirrorelement.

In some implementations, an entrance side optical surface of the beamsplitter and a side of the beam splitter facing the mirror element arealigned at an angle with respect to one another to produce apredetermined angular offset between the first and second exit beams.The beam influencing optical unit according to those implementations canbe used, for example, if the tiltability of the mirror element and/or ofthe beam splitter is limited due to limited available space. Forexample, the optical surfaces on the entrance side of the beam splitterand on the side of the beam splitter facing the mirror element canextend at an acute angle that allows an angular offset between theoptical surfaces to be produced, even if beam splitter and mirrorelement are aligned parallel to each other. The first entrance beam canbe reflected without change as the first exit beam at the entrance sideoptical surface of the beam splitter. Due to the angular offset betweenthe optical surfaces of the beam splitter, the second entrance beam canbe deflected such that the beam direction of the second exit beamchanges with respect to the beam direction of the first exit beam. Theangle between the two surfaces can be matched to the correspondinginstallation space situation.

In some implementations, the beam influencing optical unit furtherincludes a focusing element for focusing the first exit beam at a firstfocal point and the second exit beam at a second focal point. The beamsplitter and the mirror element can be used to align both exit beamswith the same focusing element and to focus them at the first and secondfocal points. The first and second exit beams, or their first and secondfocal points, can be focused in a targeted fashion in a first and secondpredetermined focus position, depending on the intended application. Thefirst and second predetermined focus positions can also coincide, ifappropriate. Reflective focusing elements (for example, parabolicmirrors) but also transmissive optical elements (for example, lenses)can be used for focusing. The beam splitter, the mirror element and thefocusing element or elements can be affixed or mounted in each case at amain structure of the beam influencing optical unit. Tiltable elements,such as, for example, the beam splitter and/or the mirror element, canbe mounted at the main structure such that they are generally driven bymotor or are manually tiltable.

In some implementations, the focusing element includes a focusing lens.A focusing lens can be used to effectively focus the two impinging exitbeams that are partially superposed on one another in particular if theystrike the lens approximately in the center. Focusing lenses that can beused include, for example, (bi)convex quartz lenses, and othermaterials, such as, for example, zinc selenide. Selection of the lensmaterial can depend on the wavelength of the radiation used.

In some implementations, the beam splitter and/or the mirror element aredisplaceable in a translational manner using, for example, actuators.Due to the translational displaceability, the relative arrangement, suchas the distance of the respective optical surfaces of the mirror elementand of the beam splitter, can be adjusted. In this way, the two exitbeams can be aligned in the direction of the focusing element, which, insome implementations, simplifies modifying the location of a respectivefocal point. The translational displaceability can also be used tomodify the angle between the first and second exit beams, such as whenthe beam splitter and/or the mirror element have a curvature. Byappropriate selection of the distance between the beam splitter and themirror element, it is possible, in some implementations, to ensure thatthe focusing element is struck substantially in the center when there isa change in the beam direction and/or divergence of the entrance beams.In an example, it is possible to compensate for a change in the beamdirection or the beam location of the second exit beam along an axisthat is perpendicular to the beam direction of the first exit beam inthe deflection plane of entrance beam and second exit beam. In order toobtain that compensation, the second exit beam in the basic position(e.g., with unchanged distance between beam splitter and mirror element)preferably has a greater distance from the entrance beams than the firstexit beam. The beam splitter and/or the mirror element are, in someimplementations, mounted at the main structure of the beam influencingoptical unit so as to be not only tiltable but also displaceable bytranslation. At least one actuator can be provided to displace the beamsplitter and/or the mirror element.

In some implementations, the beam influencing optical unit can include acontrol apparatus for controlling actuators that tilt and/or deformand/or displace the beam splitter and/or the mirror element. Due to theindividual controlling of the actuators, it is possible for the firstand second focal points to assume a predetermined (prespecified)position in three-dimensional space or to follow a predeterminedmovement path in three-dimensional space, e.g., the focus positions canbe adjusted in all three spatial dimensions (X direction, Y directionand Z direction). Optical elements that are located in the beam path ofthe two exit beams, such as, for example, the focusing element,preferably continue to be illuminated centrally by both exit beams. Bychanging the distance between the beam splitter and the mirror element,continuous illumination is possible even in the case of a change in thebeam direction of the second exit beam, if the change of the beamposition of the second exit beam takes place along an axis that isperpendicular to the beam direction of the first exit beam and islocated in the deflection plane of entrance beam and second exit beam.As discussed herein, it is preferable that the second exit beam in thebasic position (e.g., with unchanged distance between beam splitter andmirror element) has a greater distance from the entrance beam than thefirst exit beam. The distance between beam splitter and mirror elementin the basic position can be selected such that the two exit beamsstrike the focusing element substantially in the center. If the beamdirection and/or divergence of the entrance beams changes, the actuatorscan be controlled such that the exit beams strike the focusing elementapproximately in the center.

The beam shaping optical unit can also be used, for example, whencontrolling the actuators individually, in an EUV beam generatingapparatus for generating EUV radiation, in which a laser beam is guidedby a driver laser apparatus in the direction of a target position. Thebeam influencing optical unit can be integrated in the beam path betweenthe driver laser apparatus and the target position, such as, forexample, in a beam guidance apparatus or another optical apparatus.

In some implementations, in the EUV beam generating device, the laserbeam is guided by the driver laser apparatus by an element that focusesthe laser beam at the target position. A (e.g., movable) target isprovided at the target position. The target then is excited whenirradiated with the laser beam, and transitions into a plasma state sothat the target emits EUV radiation. Driver lasers, e.g., in the form ofCO₂ lasers, which permit a high conversion efficiency between the inputpower of the driver laser and the output power of the EUV radiation canbe used as driver laser apparatuses. The target materials can include,for example, tin.

In some implementations, the EUV radiation generating apparatus has abeam shaping system that includes a beam influencing optical unit andone or two radiation sources for generating the first and secondentrance beams. The first and the second entrance beam, which areapproximately collinear and, in some implementations, collimated, can begenerated, for example, by a single radiation source at the same timeand/or be amplified in additional amplifier stages suitable foramplifying a laser beam. In some implementations, the beam that isincident on the beam influencing optical unit is first split at the beamsplitter into two entrance beams that differ in terms of at least one oftheir properties (for example, in terms of wavelength and/orpolarization direction). The beam splitter can be configured such thatthe intensity of the radiation components of the two entrance beams hasa desired ratio. In some implementations, the beam influencing opticalunit can be used to modify the beam direction and/or the beam divergenceof two entrance beams to strike a movable target with both exit beamswith targeted accuracy at two predetermined positions.

The first and second entrance beams can also be generated by tworadiation sources independently of one another at the same time or witha time offset. In both cases, the first entrance beam exits the beamshaping system as the first exit beam and the second entrance beam exitsas the second exit beam. The radiation generation by the one or moreradiation sources can be pulsed or continuous. It is possible, forexample, for the entrance beams generated by the same or differentradiation sources to strike the beam influencing optical unit in quicksuccession.

In some implementations, the EUV radiation generating device includes anadjustment apparatus for adjusting the first focal point to a firstpredetermined focus location and the second focal point to a secondpredetermined focus location, in which the first and secondpredetermined focus locations can also coincide. This adjustment can, insome implementations, be advantageous if disturbances that areassociated with the radiation source or sources occur, where thedisturbances lead to a change in the focus location. For example, it ispossible for disturbances in the form of fluctuations in the directionor divergence of entrance beams (such as drift) to be “adjusted out” orsuppressed by the adjustment apparatus such that the two exit beams orthe two focal points remain in their respective predetermined positionor predetermined location by the re-adjustment.

Further advantages can be gathered from the description and the drawing.It is likewise possible for the previously mentioned features and thefeatures still to be mentioned to be used by themselves or for severalof them to be used in desired combinations. The embodiments that areillustrated and described are not to be understood as a limitingenumeration, but rather have exemplary character for the explanation ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic that illustrates a first beam influencing opticalunit having a beam splitter and a mirror element.

FIG. 2 is a schematic that illustrates a second beam influencing opticalunit having a deformable mirror element.

FIG. 3 is a schematic that illustrates a third beam influencing opticalunit, in which the beam splitter has on the entrance side a curvedoptical surface.

FIG. 4 is a schematic that illustrates a fourth beam influencing opticalunit having a plate-type, wedge-shaped beam splitter.

FIG. 5 is a schematic that illustrates a beam influencing optical unithaving a focusing lens.

FIG. 6 is a schematic that illustrates the beam influencing optical unitof FIG. 5 having a mirror element that has been moved to a differentposition.

FIG. 7 is a schematic that illustrates an EUV radiation generatingdevice having a beam influencing optical unit.

DETAILED DESCRIPTION

In the description of the drawings that follows, identical referencesigns are used for identical or functionally identical components. Nobeam refraction at the respective surfaces has been shown.

FIG. 1 is a schematic that illustrates a beam influencing optical unit 1having a beam splitter 2 that is configured to reflect a first entrancebeam 3 a as a first exit beam 4 a and to transmit a second collimatedentrance beam 3 b that is collinear with the first entrance beam 3 a.The beam splitter 2 has a plate-shape, e.g., it has a relatively lowthickness of typically less than approximately half its aperture. Thecollinear entrance beams 3 a, 3 b are incident at an angle β withrespect to the normal direction 5 of the beam splitter 2. The beaminfluencing optical unit 1 also has a mirror element 6 that is arrangedin the beam path of the transmitted, second entrance beam 3 b and isconfigured or arranged such that the second entrance beam 3 b isreflected back to the beam splitter 2 to form a second exit beam 4 bthat is transmitted by the beam splitter 2. Although FIG. 1 shows thefirst and second entrance beams 3 a, 3 b striking the beam splitter 2and intersecting completely, in actual practice, the first and secondentrance beams 3 a, 3 b strike the beam splitter 2 in partiallysuperposed fashion, e.g., their beam paths intersect only partially.

The mirror element 6 is arranged in the beam influencing optical unit 1such that it is tiltable about a tilt axis 7. The second exit beam 4 bextends at an angle α with respect to the first exit beam 4 a that isadjustable based on the alignment of the mirror element 6. Alternativelyor additionally, the beam splitter 2 can be arranged in the beaminfluencing optical unit 1 such that it is tiltable about acorresponding tilt axis (not illustrated). In this way, two collinearentrance beams 3 a, 3 b can be influenced by the beam influencingoptical unit 1 simply and effectively such that the relative beamdirection or the angle α between the corresponding exit beams 4 a, 4 bis adjustable in a targeted fashion. If appropriate, the alignment ofthe exiting laser beams 4 a, 4 b does not have to be adjustable. In thiscase, the beam splitter 2 and the mirror element 6 are arranged in afixed fashion and are aligned so they are non-parallel.

In order to tilt the mirror element 6 (or to tilt the beam splitter 2),an actuator 8 is provided, that allows the alignment or orientation ofthe mirror element 6 to be modified. As shown in FIG. 1, the actuator 8is configured as a linear actuator acting in a movement direction 9. Thelinear actuator tilts the mirror element 6, via a lever mechanism 10,about the tilt axis 7 and thus changes the exit direction of the secondexit beam 4 b relative to the first exit beam 4 a. As shown in FIG. 1,the second exit beam 4 b is at least partially superposed on the firstexit beam 4 a. The purpose of this at least partial superposition is sothat the optical units that are arranged in the further beam path of thetwo exit beams 4 a, 4 b can have a relatively small optical surface.However, a beam path of the two exit beams 4 a, 4 b without mutualsuperposition is also possible by way of a corresponding relativearrangement of the mirror element 6 with respect to the beam splitter 2.

The beam splitter 2 is configured in the example shown in FIG. 1 as awavelength-selective element that either reflects or transmits the twoentrance beams 3 a, 3 b based on their wavelengths. A front side 11 a ofthe beam splitter 2 can have, e.g., a wavelength-selective, typicallydielectric, coating. Alternatively, or in addition, the beam splitter 2can be configured as an etalon that only lets radiation that meets aspecified resonance condition and that corresponds to a definedwavelength or a defined wavelength range pass through the beam splitter2 as the second exit beam 4 b. In the case the beam splitter is anetalon, both the entrance side optical surface 11 a and the mirrorelement side optical surface 11 b can have a partially reflectivecoating that is applied onto a substrate. Alternatively, the beamsplitter 2 can be configured as a two-part etalon, e.g., the beamsplitter 2 is assembled from two plates 11 a, 11 b, between which a thingap is formed.

FIG. 2 illustrates another example of a beam influencing optical unit 1for influencing the first entrance beam 3 a and the second entrance beam3 b, in which the second entrance beam 3 b is collinear with the firstentrance beam 3 a. In contrast to FIG. 1, the mirror element 6 is notonly tiltable about the tilt axis 7, but is also configured to bedeformable for producing a varying beam divergence of the first andsecond exit beams 4 a, 4 b; more specifically, the curvature of a mirrorsurface 12 of the mirror element 6 is adjustable. If the mirror element6 is deformed, the optically effective surface 12 of the mirror element6 changes, and thus the beam divergence of the second exit beam 4 b thatis reflected by the optical surface 12 also changes. If the mirrorsurface 12 of the mirror element 6 is curved convexly toward theoutside, the divergence of the reflected, second exit beam 4 bincreases. If the curvature is reduced, the divergence of the secondexit beam 4 b also decreases. It is possible with a correspondingcurvature of the mirror surface 12 to also focus the second exit beam 4b.

To produce a curvature of the mirror surface 12 that is more or lessstrongly pronounced, a controllable actuator 13 is provided in theinterior of the mirror element 6 in the example shown in FIG. 2. Theactuator 13 can be configured, for example, as a linear actuator actingin a movement direction 14, in which the actuator 13 centrally engagesthe rear side 15 of the mirror surface 12 and thus causes the mirrorsurface 12 to curve to varying extents. Adjustable deformation orinfluencing of the curvature of the surface 12 of the mirror element 6is also possible in other ways. For example, fluid pressure can beapplied to the surface 12 of the mirror element on the rear side 15, inwhich the surface 12 is configured as a membrane, as described, forexample, in EP 1 424 584 A1.

Not only can the beam influencing optical unit 1 of FIG. 2 cause therelative direction of the exit beams 4 a, 4 b to differ by modifying thecollinear entrance beams 3 a, 3 b (e.g., the second exit beam 4 bproceeds at an angle α with respect to the first exit beam 4 a), but thebeam influencing optical unit 1 also can cause the beam divergence ofthe first and second exit beams 4 a, 4 b to differ. This is can be anadvantage, in some implementations, such as when the exit beams 4 a, 4 bare focused, since the change in beam divergence provides another degreeof freedom in fixing of the focus position.

The beam splitter 2 is configured in FIG. 2 in an exemplary manner as apolarization beam splitter. That is, the two entrance beams 3 a, 3 b arereflected or transmitted based on their polarization direction. Forexample, a polarization-selective coating can be applied on the frontside of the beam splitter 2, in which the coating permits transmissionor reflection of the entrance beams 3 a, 3 b based on the polarizationdirection. The first entrance beam 3 a having a first polarizationdirection and the second entrance beam 3 b having a second polarizationdirection that is perpendicular to the first one can be twoperpendicularly polarized radiation components of the same beam that isgenerated by a radiation source such as, for example a laser source, andthat enters the beam influencing optical unit 1. In this case, theentering beam is split at the beam splitter 2 into the two entrancebeams 3 a, 3 b. The beam splitter 2 in FIG. 2 can alternatively beconfigured as a wavelength-selective beam splitter 2 according toFIG. 1. Similarly, the beam splitter 2 in FIG. 1 can alternatively beconfigured as a polarization beam splitter.

FIG. 3 is a schematic that illustrates another example of a beaminfluencing optical unit 1. As shown in FIG. 3 the beam splitter 2 andmirror element 6, which are arranged to generate an exit angle α betweenthe first and second exit beams 4 a, 4 b, are tilted relative to oneanother (i.e., not parallel). In contrast to the beam influencingoptical units 1 of FIG. 1 and FIG. 2, the beam splitter 2 in FIG. 3 isconfigured to produce a predetermined beam divergence of the first exitbeam 4 a. To this end, the entrance side optical surface 16 of the beamsplitter 2, where, for example, a wavelength-selective coating may beprovided, has a specified convex curvature. Due to the convex curvatureof the optical surface 16, the beam divergence of the first exit beam 4a is increased. Alternatively or additionally, a mirror element sideoptical surface 17 of the beam splitter 2 can have a curvedconfiguration to modify the beam divergence of the second exit beam 4 b.The curvatures of the optical surfaces 16, 17 can be identical ordifferent. By fixing the curvatures of the optical surfaces 16, 17, itis possible to set the beam divergence of exit beams 4 a, 4 b to a fixedvalue independently of the curvature of the mirror element 6.

FIG. 4 is a schematic that illustrates another example of a beaminfluencing optical unit 1, in which an entrance side optical surface 16and a mirror element side optical surface 17 of the beam splitter 2 arealigned with one another at an angle γ (the angle γ in FIG. 4 isapproximately 10°, though larger angles are also possible) to produce apredetermined angular offset, i.e., a predetermined angle α, between thefirst and second exit beams 4 a, 4 b. Once the second exit beam 4 b haspassed through the plate, wedge-shaped beam splitter 2, the second exitbeam 4 b is tilted relative to the first exit beam 4 a such that the twoexit beams 4 a, 4 b have a specified angle α with respect to oneanother, even if the beam splitter 2 (more precisely, the central planethereof) and the mirror element 6 are aligned parallel with respect toone another. The angle α between the two exit beams 4 a, 4 b canadditionally be changed by tilting of beam splitter 2 and mirror element6, as shown, for example, in FIG. 1 to FIG. 3.

FIG. 5 and FIG. 6 are schematics that illustrate an example of a beaminfluencing optical unit 1, which includes, in addition to the beamsplitter 2 and the mirror element 6, a focusing element 18 configured asa focusing lens for focusing the first exit beam 4 a to a first focalpoint 19 a and the second exit beam 4 b to a second focal point 19 b.The first and second exit beams 4 a, 4 b are partially superposed on oneanother and can therefore be directed, by the beam splitter 2 and themirror element 6, onto the same focusing element 18. From the focusingelement 18, the first exit beam 4 a is focused to a first(predetermined) focal point 19 a and the second exit beam is focused toa second (predetermined) focal point 19 b. The beam splitter 2, themirror element 6 and the focusing element 18 are mounted at a mainstructure (not illustrated) of the beam influencing optical unit 1.

In FIG. 5 and FIG. 6, the mirror element 6 is configured to bedisplaceable in translational fashion. For illustrative purposes, inFIG. 6 the distance A′ between the beam splitter 2 and the mirrorelement 6 was increased, by way of example, relative to the distance Ashown in FIG. 5 by translational displacement of the mirror element 6 ina direction that is parallel to the incident direction 20 of theentrance beams 3 a, 3 b. Due to the translational displacement, it ispossible to readjust the distance A, A′ between the beam splitter 2 andthe mirror element 6 such that the two exit beams 4 a, 4 b can bealigned approximately centrally with the focusing element 18, even inthe case of a drift of the beam direction. If the beam splitter 2 is nottilted, as is illustrated in FIG. 5 and FIG. 6, the first focal point 19a remains in the origin of the X, Y, Z coordinate system (with the Xaxis parallel to the optical axis of the focusing element 18), whereasin FIG. 5, the second focal point 19 b is located at positive Xcoordinates and in FIG. 6 at negative X coordinates.

Alternatively or additionally, the beam splitter 2 can be displaceablein translational fashion. For the purposes of translationaldisplacement, a controllable actuator 21 that is configured as a linearactuator is provided, as shown in in FIG. 5 and FIG. 6. The beaminfluencing optical unit 1 includes a control apparatus 22 configured tocontrol the actuators 8, 13, 21, which tilt, deform and/or displace thebeam splitter 2 and/or the mirror element 6. The actuators 21 areconnected to the control apparatus 22 to allow signal exchanges for thepurpose of control. Similar connections for signal exchanges to thetilting and/or deforming actuators 8, 13, as shown in FIG. 1 and FIG. 2,are also provided, although they are not illustrated in FIG. 1 and FIG.2 for the purposes of clarity. The control apparatus 22 allows theactuators 8, 13, 21 to be individually controlled so that the first andsecond focal points 19 a, 19 b in each case can be set to a desired(predetermined) location in the three-dimensional space. For example,the individual control of the actuators 8, 13, 21 allows the location ofthe two focal points 19 a, 19 b to be set relative to one another inthree spatial dimensions (X, Y, Z).

The location of the focal points 19 a, 19 b in the direction of the Yaxis can be modified by tilting the beam splitter 2 or the mirrorelement 6 about a tilt axis that is located in the plane of the drawing(not illustrated). By changing the distance A, A′ between the beamsplitter 2 and the mirror element 6 to illuminate the focusing element18 approximately centrally, it is possible to compensate for the changein the beam location only along the X axis, provided that the secondexit beam 4 b is in a basic position (e.g., with unchanged distance Abetween beam splitter 2 and mirror element 6 (see FIG. 5)) has a largerdistance from the entrance beams 3 a, 3 b than the first exit beam 4 a.The X axis is located perpendicular to the beam direction of the firstexit beam 4 a (and typically parallel to the optical axis of thefocusing element 18) in the deflection plane of the entrance beams 3 a,3 b and the second exit beam 4 b. The ability to provide suchcompensation does not exist for a change of the beam direction along theY axis. The focus location can be changed in the Z direction by changingthe divergence.

FIG. 7 is a schematic that illustrates a beam shaping system that isconfigured as an EUV radiation generating device 30. The beam shapingsystem 30 includes a radiation source, configured as a driver laserapparatus 31, for generating the first and second entrance beams 3 a, 3b with different wavelengths. The beam shaping system 30 also includes abeam influencing optical unit 1 that is configured and arranged as inFIG. 5 and/or FIG. 6 for focusing the first and second exit beams 4 a, 4b at a first and second focus position 19 a, 19 b. The beam shapingsystem 30 also includes a beam guidance apparatus 33 having multipledeflection mirrors 34 or parabolic mirrors 35. The beam shaping system30 also includes one or more measurement apparatuses (not illustratedfor purposes of clarity) for monitoring the beam path of the entrancebeams 3 a, 3 b.

The driver laser apparatus 31 includes two CO₂ beam sources and multipleamplifiers for generating the two entrance beams 3 a, 3 b with a highradiation power (>1 kW) in pulsed form, in which the two differentwavelengths are produced by a grating in the cavity of the respectivebeam sources. The entrance beams 3 a, 3 b are guided from the driverlaser apparatus 31 first into the beam guidance apparatus 33 and then tothe beam influencing optical unit 1, which is arranged in a vacuumchamber in the example shown in FIG. 7. The entrance beams 3 a, 3 bstrike the beam splitter 2 at an angle β of approximately 45° withrespect to the normal direction 5 of the beam splitter 2 of the beaminfluencing optical unit 1. After the relative beam direction and/orbeam divergence of the two entrance beams 3 a, 3 b are modified by thebeam influencing optical unit 1 as described herein, the beams 3 a, 3 bare directed to the focusing element 18. The focusing element 18 isconfigured as a focusing lens and focuses the two entrance beams 4 a, 4b to the two focal points 19 a, 19 b.

For targeted focusing of the two exit beams 4 a, 4 b, the beam shapingsystem 30 also includes an adjustment apparatus 36 for adjusting thefirst focal point 19 a to a first predetermined focus location and thesecond focal point 19 b to a second predetermined focus location. Forthe adjustment, the beam path of the entrance beams 3 a, 3 b is measuredby means of the measurement apparatuses described herein. Forcontrolling the corresponding actuators, the adjustment apparatus 36 isconnected to the beam influencing optical unit 1 and to the measurementapparatuses (not shown).

In the beam shaping system 30 shown in FIG. 7, the predetermined focuslocation corresponds to the position of a movable target in the form ofa tin droplet, which moves along a specified path. It is possible, usingthe driver laser apparatus 31, to generate two laser pulses in quicksuccession which form the entrance beams 3 a, 3 b. To ensure that theentrance beams 3 a, 3 b, generated with the time offset, strike themoving target, they are focused at different spatial points or differentpredetermined focus locations 19 a, 19 b along the movement path of thetarget, as is indicated in FIG. 7. A tin droplet which serves as thetarget material is struck by both focused exit beams 4 a, 4 b and thustransitions into a plasma state which generates EUV radiation. Thetarget material is supplied to the beam influencing optical unit 1 withthe aid of a provision apparatus (not illustrated) that guides thetarget material along a specified path that intersects with the targetposition, more precisely the predetermined focus locations 19 a, 19 b.

In FIG. 7, the beam splitter 2 and the mirror element 6 of the beaminfluencing optical unit 1 are arranged in the beam path between thelast deflection mirror 34 of the beam guidance apparatus 33 and thefocusing element 18. The beam splitter 2 and the mirror element 6 canalso be arranged at a different location. For example, the beam splitter2 and the mirror element 6 can be arranged at the location of the lastdeflection mirror 34 of the beam guidance apparatus 30.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. Accordingly, other embodimentsare within the scope of the following claims.

What is claimed is:
 1. An extreme ultraviolet (EUV) radiation generatingdevice comprising: a radiation source operable to generate a firstentrance beam and a second entrance beam; a beam influencing opticalunit operable to modify at least one of a direction and a beamdivergence of the first entrance beam and of the second entrance beam,wherein the beam influencing optical unit comprises: a beam splitterarranged to receive the first entrance beam and the second entrance beamthat is at least partially superposed on the first entrance beam,wherein the beam splitter is configured to reflect the first entrancebeam as a first exit beam and to transmit the second entrance beam, anda mirror element arranged in the beam path of the transmitted, secondentrance beam to reflect the second entrance beam back to the beamsplitter so as to form a second exit beam that is transmitted by thebeam splitter and that is at least partially superposed on the firstexit beam, wherein the beam influencing optical unit is configured tomodify an angle between the first exit beam and the second exit beamand/or to modify a beam divergence of the first exit beam and/or of thesecond exit beam.
 2. The EUV radiation generating device of claim 1,wherein the beam splitter and/or the mirror element are tiltable, suchthat the angle between the first exit beam and the second exit beamvaries upon tilting the beam splitter and/or the mirror element.
 3. TheEUV radiation generating device of claim 1, wherein the mirror elementis operable to be deformed so that the beam divergence of the secondexit beam varies in response to a deformation of the mirror element. 4.The EUV radiation generating device of claim 1, wherein the beamsplitter comprises a shape of a plate.
 5. The EUV radiation generatingdevice of claim 1, wherein the beam splitter comprises awavelength-selective optical element.
 6. The EUV radiation generatingdevice of claim 5, wherein the beam splitter comprises an etalon.
 7. TheEUV radiation generating device of claim 1, wherein the beam splittercomprises a polarizing beam splitter.
 8. The EUV radiation generatingdevice of claim 1, wherein a first surface of the beam splittercomprises a curvature configured to produce a divergence of the firstexit beam and/or the second exit beam.
 9. The EUV radiation generatingdevice of claim 1, wherein a first side of the beam splitter, arrangedto receive the first entrance beam and the second entrance beam, and asecond opposite side of the beam splitter are aligned at a non-parallelangle with respect to one another.
 10. The EUV radiation generatingdevice of claim 1, further comprising a focusing element arranged andconfigured to focus the first exit beam and the second exit beam to afirst focal point and to a second focal point, respectively.
 11. The EUVradiation generating device of claim 10, wherein the focusing elementcomprises a focusing lens.
 12. The EUV radiation generating device ofclaim 1, further comprising an actuator arranged to displace the beamsplitter or the mirror element.
 13. The EUV radiation generating deviceof claim 1, further comprising: an actuator arranged to tilt, deformand/or displace the mirror or the beam splitter; and a control apparatusoperable to control the actuator.
 14. The EUV radiation generatingdevice of claim 1, further comprising: a focusing element arranged andconfigured to focus the first exit beam and the second exit beam to afirst focal point and to a second focal point, respectively; and anadjustment apparatus operable to adjust the first focal point to a firstpredetermined focus location and the second focal point to a secondpredetermined focus location.